The present invention relates to a process for the fluid catalytic cracking which is improved by the pre-vaporization of the hydrocarbon feed that is to be cracked in the catalytic cracking unit. More specifically, the present invention relates to a process for the catalytic cracking in a fluidized bed where the feed is completely and previously vaporized before it is made to contact the hot cracking catalyst from the regenerator vessel. In terms of the vaporization of the feed the pre-vaporization of the feed dispenses with the heat exchange with the catalyst, so that coke deposits on the catalyst particles are minimized.
Fluid catalytic cracking (FCC) is a process of paramount importance for obtaining valuable oil products such as gasoline, diesel oil and liquefied petroleum gas (LPG).
In spite of the fact that the FCC process has been known for more than 50 years, new, improved techniques are continuously sought which could improve the process, so as to increase the yield in more valuable products. Broadly speaking, it is common knowledge that the main object of the FCC processes is the maximization of more valuable products.
Generally, the feed stocks which are more usually submitted to the FCC process are those refinery streams from side cuts of vacuum towers, the so-called vacuum heavy gasoil (VHGO), or streams which are heavier than the former, from the bottom of atmospheric towers, the so-called atmospheric residue (AR), or still, mixtures of such streams. When submitted to the FCC process, such streams are made to contact a catalyst made up of fine particulate material in a conversion zone, in the absence of hydrogen, so as to be converted into lighter and more valuable hydrocarbon streams.
According to the state-of-the-art FCC process, it is only when the feed is made to contact the regenerated hot catalyst that heat transfer occurs so as to attain the required temperatures to vaporize the feed and effect the cracking reactions.
A well-known aspect of the process is the initial contact of the catalyst and the feed, which has a decisive influence on the conversion and selectivity to noble products.
Improvements in the contact between catalyst and feed have been tried, always aiming at promoting a quick vaporization of the feed and an intimate contact with the catalyst in the short time available in the reaction zone, which averages 2 seconds. In the reaction zone, the catalyst may flow upstream (riser) or downstream (downer).
In order that the catalytic reactions be processed, it is necessary that the vaporization of the feed and catalyst in the mixing area occurs in a few milliseconds, so that the molecules of vaporized hydrocarbons may contact the catalyst particles, permeate its micropores and be influenced by the acidic sites of the catalyst which cause the catalytic cracking.
In case a quick vaporization is not reached, the thermal cracking of the liquid portions of the feed is observed. It is well known that thermal cracking leads to by-products such as coke and fuel gas, mainly for residua-containing feeds. Therefore, thermal cracking on the basis of the reaction zone undesirably competes with the catalytic cracking aimed by the process.
A better vaporization will be obtained if the feed is properly vaporized so as to form a fine spray on the catalyst phase. In order to obtain such spray several models of feed atomizer in the reaction zone have been developed, for example, the ULTRAMIST atomizer developed by PETROBRAS. A really efficient system for atomizing the feed would yield a spray having particles fine enough so that 90% of the feed could be vaporized in less than 0.1 seconds, preferably in less than 0.05 seconds, considered from the moment of the injection of the feed in the reaction zone.
A further important parameter in the atomization of the feed is the temperature of the feed in the atomizer. The higher the temperature of the feed in the atomizer, the higher will be the surface area of the droplets in the spray, and therefore the higher will be the contact area between the feed and the catalyst, with significant impact on the ease of vaporization.
For the residual feeds used in the FCC process and for the temperature ranges practiced, it can be seen that the increase in the contact area by using higher temperatures of the feed may attain 30%.
In the industrial furnaces used for pre-heating the feed of the FCC unit, the temperature is limited to 300xc2x0 C., which is insufficient to vaporize it. It is obvious that the feeds of the FCC unit cannot be indefinitely heated since the excessive heating would cause the thermal cracking and the coking of the furnaces.
Therefore, although desirable, the complete vaporization of the feed does not occur in an efficient way in the conventional FCC units, mainly in those, which operate with heavy gasoil or atmospheric residua, or even with a mixture of these feeds. The contact of the feed, which is not completely vaporized, with the hot catalyst eventually results in the deposition of liquid droplets of the feed on the surface of catalyst particles, so that coke production is increased.
Only the pre-vaporization of the feed to the FCC unit would prevent the deposition of liquid on the catalyst particles and eliminate the negative effects of such deposition. The specialized literature, however, has not focused this matter, the subject of choice being the atomization or the pre-treatment of the feed.
One point, which has not received much attention from the experts in the fluid catalytic area, is that if the heating of the feed up to the point of complete vaporization before contacting the regenerated catalyst could bring further benefits to the FCC process.
The complete vaporization of the feed seems to be actually necessary. A few studies show that, even at the high temperatures found in the reaction zone, the feed may xe2x80x9cwetxe2x80x9d the hot catalyst particles and be sucked by its pores through a capillary action, whereby coke is formed. The estimated time of suction, through capillary action of the liquid present in the surface into the interior of the pores is of the order of 0.1 seconds, this figure being frequently lower than the time required for the complete vaporization of the liquid.
G. Yang et al. in xe2x80x9cSome Fundamental Aspects of Residuum Catalytic Crackingxe2x80x9d, Acta Petrolei Sinica (Petroleum Processing Section), October, 1997, pages 12-16, collected catalyst samples at various heights of the reaction zone (riser) of an industrial FCC unit and assessed the carbon content and the microactivity (MAT) of such samples. The results indicate that at 1.0 meter height up to the point of feed injection, the carbon content rises steeply from 0.2 to 2.71 weight %, while MAT activity decreases from 65.9 to 28.1 percent. The coke content of samples from a higher point, at 3.3 meters in the reaction zone (riser) decreases up to 1.01 weight %, the MAT activity is partially recovered, and attains 45.9%. Above this point, carbon and MAT activity are only slightly altered.
To explain these results, the experts assumed that the catalyst coverage by carbon and the filling of the catalyst pores with droplets of liquid feed lower the catalyst activity, the portion of the feed, which is not vaporized being considered as coke. The reactions of the non-vaporized feed proceeding throughout the reaction zone (riser), the catalyst surface and the MAT activity are in part recovered.
Therefore, these observations have led the Applicant to infer that the complete vaporization of the feed of the FCC unit, before the contact with the regenerated catalyst may bring additional benefits to the FCC process, besides those obtained by just the increase in the feed temperature.
However, in spite of several clues as to the importance of the complete vaporization of the feed for the FCC units, the patent literature focuses the solution of the problem of heavy feed cracking on other issues.
U.S. Pat. No. 5,324,418 and U.S. Pat. No. 5,324,419 suggest to make closer the temperature of the regenerated catalyst and the feed by pre-heating the feed at high temperatures (around 426xc2x0 C.), the use of catalyst coolers (catcoolers) and high catalyst/oil ratios to define a new operation window in a FCC unit where the product yields would be optimized.
U.S. Pat. No. 5,271,826 teaches an FCC process which is typical of heavy feeds in which the feed is heated through the contact with the hot catalyst at a temperature of ca. 565xc2x0 C. so as to promote the thermal as well as the catalytic cracking of the feed, and adopts a quick cooling or quench proceeding with a lighter feed in an intermediate section of the reaction zone (riser) to quickly cool the mixture and catalytically control the cracking process.
U.S. Pat. No. 4,980,051 teaches a process for the pre-treatment of a residuum-containing feed in which the heating of the feed to the FCC unit occurs through direct transfer of heat through the contact between the feed and the reaction products. However, the thermal exchange through direct contact only heats the feed before the contact with the catalyst.
WO patent 88/01638 teaches that it is possible to pre-treat heavy hydrocarbon feeds lean in hydrogen such as atmospheric residua by using state-of-the-art processes such as visbreaking or hydrotreating before the catalytic cracking process. There are also comments on the use of non-conventional sources of energy, such as a microwave generator, for heating the feed. In spite of the relatively higher price, these non-conventional energy sources may heat preferably different portions of the heavy hydrocarbon feed, being more selective than a conventional thermal pre-treatment.
WO patent 85/4182 also describes the pre-treatment of the feed before submitting it to the FCC process. The pre-treatment aims at substantially lowering the contents of heavy metal contaminants as well as most of the coke precursors. The pre-treatment of the feed is coupled in the vicinity of the catalytic cracking reactor such as the product which leaves the pre-treatment system is in the gas phase, at a temperature between 371 and 593xc2x0 C. In view of the reduction in coke precursors effected in the feed, such an arrangement allows a more efficient use of energy and a better catalytic cracking.
Thus, the patent literature indicates that the issue of the cracking of heavy feeds has been addressed by indirect ways, such as the pre-treatment of the feeds, with separation of the coke precursors previous to submitting the feeds to the FCC process. However, this approach involves the extra cost of a further unit coupled to the FCC unit, which may not be economically interesting to the refiner.
Another approach to the heavy feed cracking is presented in U.S. Pat. No. 4,985,136. This patent teaches an FCC unit, which operates under crossed flow between a downward catalyst panel and the hydrocarbon feed. The main feature of this publication is the extremely low contact time between the feed and the regenerated catalyst, which under optimum conditions will be lower than 0.5 seconds, more preferably between 0.1 to 0.2 seconds. The main advantage of this system would be to eliminate back mixing of the catalystxe2x80x94which is colderxe2x80x94with the feed or with reaction products, so as to avoid secondary, undesirable side reactions, which yield less noble products.
However, the point which could be considered as an advantage of the cited U.S. document, that is, the very low contact time, is actually a drawback of the technology described therein. An extremely low contact time will certainly be lower than the time required for the vaporization of the feed, this leading to large amounts of coke on the catalyst since, still according to the teachings of U.S. Pat. No. 4,985,136, the vaporization of the feed would be obtained by direct heating with the regenerated catalyst. However, as is well known by the experts, the complete vaporization of heavy feeds such as those described in that European document is not so immediate nor so efficient as desired, the point of the vaporization of the feed being approached only under conventional ways, that is, through heat exchange with the hot regenerated catalyst.
For the new FCC units operating at extremely low contact times between catalyst and feed, it is probable that the pre-vaporization of the feed turns out to be a very important feature, perhaps it constitutes even the necessary condition to allow the operation under such reduced contact times.
On evaluating the pre-vaporization of the feed to the FCC unit, another important feature is the adjustment of the heat balance of the unit. As the FCC unit operates under heat balance, it is obvious to the experts that the increase in the temperature of the feed entails the modification of the flow rate of regenerated catalyst to the reaction zone or still the temperature of the regenerator in order to keep the heat balance of the unit.
In order to keep the heat balance of the unit in spite of the increase of the temperature of the feed, the circulation of regenerated catalyst to the reaction zone should be reduced, as a result of the reduction of heat requirement. Even in the case the temperature of the regenerator were kept constant with the aid of a catalyst cooler, still the circulation of regenerated catalyst to the reaction zone should be reduced. In order to keep the same circulation of regenerated catalyst, the regenerator temperature should be reduced to figures much lower than those required to a proper regeneration of the catalyst (that is, coke content lower than 0.05%), with obvious losses for the refiner.
One approach to the adjustment of the thermal balance of the FCC unit is described in EPC patent application 97307435.4 (USA application 199708/933,006) of the Applicant and herein fully incorporated as reference, which consists in rendering possible that the temperature of the feed varies by keeping the regeneration temperature constant and at the ideal value. Contrary to the state-of-the-art techniques, the suggested technique keeps also constant and at the ideal values the circulation of regenerated catalyst. This way the operation of the FCC unit with pre-vaporized feed is rendered fully operative, without any harm to the catalyst/oil ratio.
Thus, the question of the pre-vaporization of the feed to the FCC unit has not been the object of the experts efforts as is proposed in the present application, which suggests as a solution for the cracking of heavy feeds in a FCC unit, the pre-vaporization of the feed before the contact with the regenerated catalyst, so as to obtain better yields than in the FCC units which operate in the conventional mode as well as making possible that such units operate under extremely low contact time.
The present invention comprises a process for the fluid catalytic cracking designed for the cracking of hydrocarbon feeds which are normally used in the refinery, besides heavier feeds, that is, those where the boiling point of a substantial portion of the feed is above 580xc2x0 C. The invention makes possible a higher gasoline production, this entailing better economics of the process.
The process of the present invention for the fluid catalytic cracking of hydrocarbon feeds, under conditions of fluid catalytic cracking, in the absence of added hydrogen, and operating under a heat balance regimen comprises the following steps:
Pre-vaporizing the feed under conditions of temperature and pressure required to promote the optimized vaporization of the feed prior to the contact with the hot regenerated catalyst;
In the reaction zone, make to contact the pre-vaporized feed with the catalyst stream from the regenerator so as to promote the catalytic cracking reactions in the vapor phase under reduced deposition of coke on the catalyst, whereby is obtained a mixture of cracked hydrocarbons and spent catalyst;
From the so-obtained mixture, separating a stream of spent catalyst, with the aid of suitable devices placed after the reaction zone, and a stream of cracked hydrocarbons;
Directing the spent and separated catalyst from the previous step to a stripping zone, then to a regeneration zone and effect the combustion of the coke deposited on the catalyst particles, so as to yield particles of regenerated catalyst whose activity is higher than that of the spent catalyst;
Directing the stream of cracked hydrocarbons to the separation and fractionation zone, so as to separate the various products obtained according to their boiling points;
Directing the stream of regenerated catalyst back to the reaction zone so as to continue the process of fluid catalytic cracking.
Thus, the present invention provides for a FCC process for the cracking of hydrocarbon feeds having high-end boiling point where the production of gasoline is increased relative to state-of-the-art techniques.
The present invention provides further a FCC process which operates with the pre-vaporized feed before the contact with the catalyst so as to dispense with the use of atomizers for the feed, since said feed is already completely vaporized this rendering possible to work under extremely low contact times between feed and catalyst since a period of time for vaporizing the feed will no longer be required.
The present invention provides further a FCC process which operates with the pre-vaporized feed in which the amount of heat transferred from the catalyst to the feed is that required just for facing energy requirements of the cracking process, which is endothermic, the energy expenditure brought by the catalyst for vaporizing the feed being no longer required.
The present invention allows the optimization of the regenerator temperature as well as of the catalyst flow rate to the reaction zone, this rendering possible the control of the circulation of regenerated catalyst independently from the feed temperature which is already pre-vaporized.
The operation of a FCC unit with a pre-vaporized feed allows the work of the regenerator at temperatures lower than those employed in conventional FCC units which process heavy feeds, resulting in lower catalyst deactivation and consequently in an operation which is economically more advantageous to the refiner.
The present invention provides further a process for the pre-vaporization of the feed where the energy required for pre-vaporization may be partially or completely provided by the heat exchange between the hydrocarbon feed and the catalyst cooler, so as to optimize the unit costs.